1. Field of the Invention
The present invention relates to an illuminating optical apparatus for uniformly illuminating a plane to be illuminated and more particularly to such illuminating optical apparatus adapted for use in an exposure apparatus for semiconductor device manufacture.
2. Related Background Art
Conventionally there is known, for example, an illuminating optical apparatus applied to a semiconductor exposure apparatus as illustrated in FIG. 12. As shown therein, the light beam from a light source 1 such as a mercury arc lamp is condensed by an elliptical mirror 2, then converted into a parallel light beam by a mirror M1 and a collimating lens 3, and guided to a fly's eye lens 4, whereby there is formed a secondary light source consisting of a plurality of secondary light source images. The light beams from said secondary light source pass through a diaphragm 5 and are condensed by a mirror M2 and a condenser lens 6 and uniformly illuminate, in superposed manner, a reticle R constituting the object to be illuminated.
The illuminating optical apparatus of the above-explained configuration projects the circuit pattern, formed on the reticle R, onto a wafer W placed on a wafer stage 8, in reduced size through a projection optical system 7.
In recent years, it is strongly desired to transfer a finer pattern onto the wafer, and, for this purpose there is conceived an improvement in the resolving power of the projection optical system. Such improvement in the resolving power may be achieved by the use of a light source of a shorter wavelength, or by an increase in the numerical aperture of the projection optical system.
However, it is difficult to construct a projection optical system matching such light source of shorter wavelength, because of lack of suitable optical material usable as transmissive optical elements.
An increase in the numerical aperture decreases the depth of focus in proportion to the square of said numerical aperture. Thus the resolving power currently achieved by the projection optical system has reached its limit.
Under such situation, there has recently been proposed so-called oblique illumination technology, which illuminates the reticle R in oblique manner by deforming the shape of the secondary light source, formed at the exit side of the fly's eye lens as shown in FIG. 12, thereby achieving a resolving power and a depth of focus significantly better than those inherent to the projection optical system 7, and such technology is currently attracting great attention.
Among said technology, there is known, for example, the annular illumination method in which an annular (doughnut-shaped) aperture is formed in the diaphragm 5, provided at the exit side of the fly's eye lens as shown in FIG. 12, to form an annular secondary light source for obliquely illuminating the reticle R, thereby achieving improvements in the resolving power and in the depth of focus.
Also there is known, as disclosed in the Japanese Patent Laid-Open Application No. 4-101148, a special oblique illumination method in which the diaphragm 5 as shown in FIG. 12 is provided with two or four apertures to form two or four secondary light sources for oblique illumination of the reticle R, thereby achieving a finer resolution and a larger depth of focus than in the annular illumination method.
Although the above-explained oblique illumination technology can provide significant improvements in the resolving power and the depth of focus over those inherent to the projection optical system, it is associated with a drawback of significantly lowered illuminating efficiency, because the light beam has to be considerably intercepted by the diaphragm provided at the exit side of the fly's eye lens in order to deform the secondary light source. This leads to a serious drawback of significant loss in the throughput.